1. Technical Field of the Invention
The present invention relates generally to a discharge lamp driver circuit working to turn on a discharge lamp, and more particularly to a noise canceller structure of such a discharge lamp driver circuit which is designed to minimize radiation of noises arising from a switching operation of the driver circuit.
2. Background Art
FIG. 7 shows a typical discharge lamp driver circuit 100 for automotive vehicles which includes a filter circuit 110, a DC/DC converter 120, an inverter 130, and a control circuit 150. The discharge lamp driver circuit 100 works to step up a dc voltage supplied from a storage battery 10 through the DC/DC converter 120 when a lighting switch 20 is turned on and converts it into an ac voltage through the inverter 130 to initiate a discharge in a lamp 30.
The lamp 30 is a discharge lamp such as a metal halide lamp typically used as a headlamp of the vehicle. Starting the lamp 30 is achieved by inducing a dielectric breakdown through a transformer (not shown) of a starter circuit to develop a high voltage between electrodes of the lamp 30. After the dielectric breakdown, the status of the lamp 30 is shifted from a glow discharge to an arc discharge to keep the lamp 30 lightened stably.
The filter circuit 110 consists of a coil 111, a capacitor 112, and a capacitor 113 and works as a noise filter.
The DC/DC converter 120 consists of a transformer 121 made up of a primary winding 121a connected to the battery 10 and a secondary winding 121b connected to the lamp 30, a MOS transistor (field-effect transistor) 122 connected to the primary winding 121a, rectifier diode 123, and a smoothing capacitor 124 and works to step up and output the voltage from the battery 10. Specifically, when the MOS transistor 122 is turned on, it will cause a primary current to flow through the primary winding 121a so that energy is accumulated in the primary winding 121a. When the MOS transistor 122 is turned off, it will cause the energy in the primary winding 121a to be supplied to the secondary winding 121b. Such turning on and off the MOS transistor 122 is repeated, thereby causing a high voltage to be outputted from a junction of the diode 123 and the smoothing capacitor 124. The transformer 121 may alternatively be so constructed that the primary and secondary windings 121a and 121b are electrically connected to each other.
The inverter 130 includes MOS transistors (not shown) arrayed in the form of an H-bridge which work to provide the ac current for turning on the lamp 30.
The control circuit 150 is responsive to a signal (lamp power signal) provided by a power detector (not shown) as functions of a lamp current and a lamp voltage to control the MOS transistor 122 in a PWM mode so as to bring the lamp power into agreement with a maximum (e.g., 65 W) when turning on the lamp 30 and with a constant power (e.g., 35 W) subsequently.
The control circuit 150 consists of a gate control circuit 150a controlling the on-off operation of the MOS transistor 122 in the PWM mode, the power detector detecting the lamp voltage, and a lamp power control circuit (not shown) controlling the lamp power to bring it into agreement with a target one based on the detected lamp current and voltage.
In operation, when the lighting switch 20 has been turned on, and the control circuit 150 has started to control the MOS transistor 122 in the PWM mode, the DC/DC converter 120 outputs the voltage produced by stepping up the voltage of the battery 10 through the transformer 121. The high-voltage produced by the DC/DC converter 120 (300V to 500V in the course of preparation for turning on the lamp 30, and about 100V after turning on the lamp 30) is further stepped up to, for example, 25 kV through the inverter 130 so that the dielectric breakdown may occur in the transformer of the starter circuit and applied to the lamp 30. This causes the lamp 30 to be turned on. After turning on the lamp 30, the polarity of the voltage to be outputted by the inverter 130 is reversed cyclically to provide the ac voltage to the lamp 30.
The above structure of the discharge lamp driver circuit 100 has a drawback in that interrupted currents arising from the on and off operations of the MOS transistor 122 of the DC/DC converter 120 to step up the voltage of the battery 10 result in radiation of noises.
The interrupted currents flow through three electrical loops: a first electrical path Lp1 extending from the capacitor 113 through a power source positive line to the primary winding 121a of the transformer 121 to a drain and a source of the MOS transistor 122 and back to the capacitor 113 through a ground line, a second electrical path Lp2 extending from the rectifier diode 123 to the smoothing capacitor 124 to the ground line to the secondary winding 121b and back to the rectifier diode 123, and a third electrical path Lp3 extending from the gate control circuit 150a to the gate of the MOS transistor 122 to the ground line and back to the gate control circuit 150a. The first, second, and third electrical paths Lp1, Lp2, and Lp3 carry currents i1, i2, and i2 arising from the on and off operations of the MOS transistor 122 by the gate control circuit 150a. 
Particularly, in a case where the above structure of the discharge lamp driver circuit 100 is installed in an automotive vehicle for lighting headlamps, when a traffic light has changed to red, and the vehicle has stopped close to an antenna installed in the rear of a preceding vehicle, it may cause electric noises to be radiated forwardly, which raise a radio disturbance in the preceding vehicle.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a discharge lamp driver circuit designed to minimize adverse effects caused by interrupted currents produced in the driver circuit.
According to one aspect of the invention, there is provided a discharge lamp driver circuit which may be employed in turning on a discharge lamp as used as a headlamp of automotive vehicles. The discharge lamp driver circuit comprises: (a) a power supply circuit connected to a de power supply; and (b) a field canceller. The power supply circuit includes a switching element and performs an on-off operation on the switching element to step up a dc voltage from the dc power supply and provide the stepped up dc voltage for turning on a discharge lamp. The power supply circuit includes an electrical path through which an interrupted current arising from the on-off operation of the switching element flows. The field canceller includes an electrical line through which the same interrupted current as that flowing through the electrical path of the power supply circuit flows, thereby producing a field canceling a field caused by flow of the interrupted current through the electrical path. This causes electrical noises radiated outside from the electrical path of the power supply circuit to be eliminated.
In the preferred mode of the invention, the power supply circuit includes a DC/DC converter. The DC/DC converter consists of a transformer made up of a primary winding connected to the dc power supply and a secondary winding connected to the discharge lamp and the switching element and works to turning on and off the switching element to provide the stepped up dc voltage to the discharge lamp through the transformer.
The electrical line of the field canceller is connected in series with the electrical path of the power supply circuit and extends so as to have the interrupted current bypass the electrical path in an orientation opposite flow of the interrupted current through the electrical path. This causes the field to be produced by the field canceller which is identical in strength and 180xc2x0 out of phase with the field arising from the interrupted current flowing through the electrical path.
The electrical line of the field canceller may be so geometrically shaped as to have an area surrounded by the electrical line which is substantially identical with an area surrounded by the electrical path of the power supply circuit. The field provided by the field canceller will, thus, be identical in strength with that produced around the electric path, thereby resulting in complete cancellation of noises arising from the field produced around the electrical path.
The electrical line of the field canceller may be laid over the electrical path of the power supply circuit so that a magnetic flux produced by the electric line of the field canceller overlaps a magnetic flux produced by the electrical path of the power supply circuit, thereby promoting cancellation of noises arising from the field produced around the electric path.
The field canceller includes an insert molded body within which the electrical line is disposed.
The field canceller may be implemented by a flexible substrate.